Plant growth substances occupy an important place in the growth and developmental processes of all plant species. The pioneers in plant growth substances, Charles Darwin, Boycen-Jensen, and others, recognized that plant growth phenomenon was under control of some chemical substances produced by the plants and in 1928 F. W. Went successfully demonstrated the existence of growth-regulating substances in plants. These compounds are useful for altering a plant's life processes or structure in some beneficial way so as to enhance yield, improve quality or facilitate harvesting. The plant growth hormones, auxins from oat seedlings, and gibberellins from a fungus, and several secondary plant products such as phenolics, lipids, steroids and terpenoids were shown to be responsible for plant growth and development. One class of plant hormone, auxins, and their synthetic mimics are of particular interest. Auxin-like activity is known to affect a number of plant processes, such as cell division, rooting at the basal end of shoots, shoot elongation, apical dominance, phototropic responses and control of abscission of organs such as buds, flowers, fruits, leaves and the like. Some of the latter elicit growth responses in conjunction with the endogenous growth hormones. Certain synthetic compounds, although different than the natural growth substances, also induce similar biological responses. Synthetic polyhydroxylated steroidal lactones are found to be highly effective plant growth promoting substances (Thompson et al., J. Org. Chem., 44, pp. 5002 to 5004, 1979; Thompson et al.—U.S. Pat. No. 4,346,226). Several oligosaccharins, brassinolides and jasmonates have been reported as non-traditional regulators of plant growth, development and gene expression (Clouse S. D. (1996), Plant J., 10, pp. 1 to 8).
Plant-growth regulators with auxin-like activity comprise an important class of chemicals for use in agriculture. As of 1994, there were approximately 29 compounds with auxin-like activity approved for agricultural use worldwide. Of these compounds, 21 were approved for use in the United States (Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Kroschwitz et al., Eds., John Wiley & Sons, New York, 1994). A particularly widely used synthetic auxin is 2,4-dichlorophenoxyacetic acid (2,4-D). Among its many uses, 2,4-D is sprayed on the foliage of citrus trees in California and Florida (also in citrus growing countries such as Israel, Spain, Morocco, South Africa, etc.) to prevent pre-harvest fruit drop and to increase fruit size.
The agricultural application of exogenous chemicals to food crops is coming under increased scrutiny by many segments of society including the agricultural industry, advocates for agricultural laborers, environmental groups and consumers. In the United States, agricultural industry concerns stem from the fact that plant growth regulators must be officially registered with the Environmental Protection Agency (EPA) before they can be used or sold. Additionally, as plant-growth regulators are often applied closer to harvest than are pesticides, the actual practical requirements for their safety are more stringent.
The screening process associated with the official registration is both time-consuming and expensive. This process includes evaluation of a plant-growth regulator's safety hazards to humans, the environment and non-target species. Further, acute and chronic toxicity must be determined. The agricultural industry shoulders a portion of the costs of the pre-registration program in higher prices. Further, the industry typically bears part of the financial burden for re-registering the compound for a particular use. For example, the California citrus industry paid approximately two million dollars to effect the re-registration of 2,4-D as a pre-harvest fruit drop inhibitor. In addition to these financial concerns, the potential toxicity of synthetic plant-growth regulators raises additional concerns regarding the safety of their use.
The concerns of environmental groups, advocates for agricultural laborers and consumers arise from the potential toxicity of plant-growth regulators. For example, auxin mimics such as 2,4-D and related phenoxy acids have moderately acute toxicity and are moderate in their local effects upon the skin or eyes. Results of cytogenic studies in Sweden indicate that, in practice, 2,4-D constitutes a cytogenic hazard to man. Additionally, 2,4-D has been found to exhibit central nervous system toxicity.
In light of their utility in preventing pre-harvest fruit drop and increasing fruit size, coupled with the expense of registration, re-registration and the potential toxicity of auxin mimics, alternatives to the use of synthetic auxins are being actively sought.
The application to plants and soils of natural auxins and natural auxin precursors is a particularly promising alternative to the use of synthetic auxin mimics. For instance, L-tryptophan has been reported to serve as precursor for the microbial formation of indole-3-acetic acid (IAA) (e.g., Arshad and Frankenberger, Plant Soil, 133, pp. 1 to 8 (1991)). Further, the synthesis of IAA upon application of tryptophan to soil has been shown to promote plant growth. For instance, growth of Douglas fir was increased by application of tryptophan and inoculation with a fungus capable of producing IAA from tryptophan. When tryptophan was applied to soils under aseptic conditions (i.e., steam-sterilized soil), L-TRP conversion to IAA was not observed (Martens and Frankenberger, Soil Science, 155, pp. 263 to 271 (1993)). Thus, it was concluded that the conversion of tryptophan to IAA was a microbe-mediated process. Certain synthetic compounds, although different than the natural growth substances, also induce similar biological responses. Synthetic polyhydroxylated steroidal lactones are found to be highly effective plant growth promoting substances. Several oligosaccharins, brassinolides and jasmonates have been reported as non-traditional regulators of plant growth, development and gene expression. An efficient and novel plant system as biosensor for detecting the environmental hazards and bioactive molecules through distinct responses has been developed at CIMAP and is being used for testing new molecules for their biological activities including plant growth regulators.